Ballast circuit for operating a gas discharge lamp

ABSTRACT

Ballast circuit for operating a gas discharge lamp, comprising a half-bridge DC-AC converter having a voltage controlled oscillator (VCO) for alternately switching the switches (T 1 ; T 2 ) of said half-bridge, said oscillator (VCO) having an input with a control voltage (V vco) which determines the operating frequency of said half-bridge, a resonance circuit connected to said half-bridge for feeding the lamp, and a feedback circuit connected at a first end to said resonance circuit for adjusting the operating frequency of said half-bridge, wherein the other end of said feedback circuit is connected to the input of said voltage controlled oscillator (VCO) and designed such that during at least a substantial part of the start-up period of the lamp an equilibrium exists wherein the half-bridge frequency is at least nearly equal to the resonance frequency and the half-bridge voltage is forced to operate at least nearly in phase with the half-bridge current.

FIELD OF THE INVENTION

The invention relates to a ballast circuit for operating a gas dischargelamp, comprising a half-bridge DC-AC converter having a voltagecontrolled oscillator for alternately switching the switches of saidhalf-bridge, said oscillator having an input with a control voltagewhich determines the operating frequency of said half-bridge, aresonance circuit connected to said half-bridge for feeding the lamp,and a feedback circuit connected at one end to said resonance circuitfor adjusting the operating frequency of said half-bridge.

BACKGROUND OF THE INVENTION

Such a ballast circuit is described in U.S. Pat. No. 5,723,953.

For igniting a (compact) fluorescent lamp two methods can be applied:warm start or cold start. Warm start means that during a specific timethe electrodes are pre-heated while maintaining the lamp voltage belowits ignition voltage. Because of the high electrode temperature theelectrodes will start emitting electrons. When after pre-heating a lampvoltage is applied which is larger than the ignition voltage, anavalanche will take place and the fluorescent lamp will be on. Aselectrons are already available, the voltage across the electrodes islow during and after the ignition process, so no sputtering of theelectrodes will occur, that is reflected in a high switching lifetime ofthe lamp.

For a cold start a high voltage is applied across the fluorescent lampin a very short time. After ignition the electrodes needs to supplyelectrons to establish a lamp current. However the electrodes are coldand the only manner to generate electrons is to force them out of theelectrode lattice by a high electric field. This high voltage will heatthe electrode and eventually thermal emission will take place. In thetime frame wherein the electrode voltage is high, the so called glowphase, sputtering of the electrode will take place that is reflected ina relatively short switching lifetime. The lamp driver should take carethat the time frame wherein the electrode voltage is high is as short aspossible. This means that in the glow phase maximum power should bedelivered to the lamp to heat up the electrodes as quickly as possible.The lamp voltage during the glow phase is high (typically 500V,depending on the lamp type). The reason to apply the cold startmechanism is to minimize the costs of the ballast.

A lamp driver usually consists of a half-bridge topology. Thepre-heating, ignition and burning states are obtained by sweeping downthe frequency of the half-bridge switches over the resonance curve ofthe resonance (LC) network. The resonance frequency(1/(2*pi*sqrt(Llamp*Clamp)) often is chosen near the start frequency(the maximum frequency) for lowest current stress during ignition.Sweeping is often established via a voltage controlled oscillator.

For cold start the frequency sweep from the start frequency to thenominal frequency (the minimum frequency) is very short, for instance 10ms, with respect to the glow time, which is more than 100 ms. Thereforewithout any measures the half-bridge will run at nominal frequencyduring the glow phase. Because this nominal frequency is far below theresonance frequency the half-bridge is not capable anymore of generatingthe high glow voltage, and furthermore the half bridge is also runningin capacitive mode. As a result the lamp may extinguish, oralternatively remain in glow mode and be destroyed thereby. Known coldstart lamp drivers which address this problem have non-integratedself-oscillating circuits which direct itself to resonance and therebymaximum power to the lamp in the glow phase. These circuits are howeverexpensive and hard to integrate in an IC.

SUMMARY OF THE INVENTION

The aim of the invention is to provide a cheap and efficient integratedballast circuit for operating a gas discharge lamp, which controls aresonant half-bridge lamp driver for maximum power during the glowphase.

Therefore the other end of said feedback circuit is connected to theinput of said voltage controlled oscillator and designed such thatduring at least a substantial part of the start-up period of the lampwherein the half-bridge frequency is at least nearly equal to theresonance frequency the half-bridge voltage is forced to operate atleast nearly in phase with the half-bridge current. Said feedback loopthereby automatically maintains the ballast at resonance frequency, andthereby at maximum power, from the moment the frequency down sweepreaches said resonance frequency until the lamp is on.

Preferably the first end of the feedback circuit is connected to theserial connection between the two switches of the half-bridge. Alsopreferably said voltage controlled oscillator input is further connectedto a current source and a capacitor, wherein said equilibrium isdetermined by said current source charging said capacitor, and saidfeedback circuit at least partially discharging said capacitor eachhalf-bridge switching cycle.

The ballast circuit described herein is in particular suited to beintegrated in an IC.

The invention furthermore relates to a lamp driver comprising saidballast circuit.

BRIEF DESCRIPTION OF THE INVENTION

The invention will now be explained in more detail with respect to thedrawings, which show an exemplary embodiment of the invention merely forthe purpose of illustration.

FIG. 1 schematically shows a conventional ballast circuit;

FIG. 2 schematically shows a ballast circuit according the invention;

FIG. 3 shows a time plot of a the half-bridge voltage of a ballastcircuit operating in inductive mode; and

FIG. 4 shows a time plot of the half-bridge voltage of a ballast circuitaccording the invention operating in near-resonance mode.

DETAIL DESCRIPTION OF THE INVENTION

According to FIG. 1 a typical ballast circuit for driving a gasdischarge lamp comprises a DC voltage terminal (vcc) and a groundterminal (gnd), a compact fluorescent lamp, capacitors C_lamp, C_dc1,C_dc2 and C-dvdt, and a coil L_lamp. Furthermore the ballast circuitcomprises a half-bridge DC-AC converter, consisting of two mosfetswitches T1 and T2, which are switched by a voltage controlledoscillator VCO. The switching frequency of oscillator VCO is determinedby an input voltage V_vco, wherein the frequency is highest if saidinput voltage is low (for instance 0 V), and lowest if said inputvoltage is high.

The man skilled in the art will appreciate that the resonance circuit ofFIG. 1 is shown for illustration purposes and may have any othersuitable configuration without departing from the scope of theinvention.

According to FIG. 2 a feedback circuit is added to the ballast circuitof FIG. 1. One end of the feedback circuit is connected to a node HBlocated between the two switches T1 and T2 of the half-bridge. The otherend of the feedback circuit is connected to the control voltage input ofthe voltage controlled oscillator VCO. The feedback circuit comprisescapacitors C_sense, a switch T4 and a transistor T3, T2 and T4 arecoupled such that T4 is on if T2 is off, and vice versa.

The frequency down sweep of the voltage controlled oscillator VCO isachieved by a current source J_0 and capacitor C_sweep, in between whichthe VCO input is connected. When the ballast is switched on the currentsource J_0 starts to load capacitor C_sweep and thereby the controlvoltage V_vco rises while the switching frequency of the VCO goes down,thereby approaching the resonance frequency of the resonance circuit. Aslong as the operating frequency is (much) higher than the resonancefrequency the resonance circuit is operating in inductive mode, asreflected in FIG. 3. In that case the voltage across T2 is zero when T2is switched on.

However, near resonance the phase angle between the half-bridge currentand the half-bridge voltage becomes so small that, given a fixed deadtime DT, wherein both T1 and T2 are off, the half-bridge voltage startsto swing back during the dead time, as shown in FIG. 4. Therefore, whenT2 is switched on there is a negative voltage step VHard at thehalf-bridge node HB. This results in discharging C_sweep via T3 andC_sense, and thereby a lower control voltage V_vco and a higheroperating frequency.

These two opposite forces, i.e. the charging of C_sweep by currentsource J_0 and the partial discharging of C_sweep when T2 is switchedon, forces the ballast towards an equilibrium wherein V-vco remainsconstant, and thus the ballast operates at near-resonance frequencywherein the half-bridge current and the half-bridge voltage are (nearly)in phase and maximum power is fed to the lamp until the lamp is on.

The amount of charge involved in the discharging of C_sweep equalsVhard*C_sense. The equilibrium thus exists when:(J _(—)0*T)−(Vhard*C_sense)=0Given the frequency f=1/T, Vhard can be expressed as follows:Vhard=J _(—)0/(f*C_sense)For example, if J_0=250 nA, f=50 kHz and C_sense=330 fF, then Vhard=15V. So the system controls itself so close to resonance that just beforeswitching on the lowside/highside power the drain source voltage equals15 V. Thus no dedicated control loop is necessary.

1. Ballast circuit for operating gas discharge lamp, comprising a half-bridge DC-AC converter having a voltage controlled oscillator for alternating switching two switches of said half-bridge, said oscillator having an input with a control voltage which determines an operating frequency of said half-bridge; a resonance circuit connected to said half-bridge for feeding the lamp; and a feedback circuit connected at a first end to said resonance circuit for adjusting the operating frequency of said half-bridge, characterized in that the other end of said feedback circuit is connected to the input of said voltage controlled oscillator and designed such that during at least a substantial part of a start-up period of the lamp an equilibrium exists wherein the half-bridge frequency is at least nearly equal to a resonance frequency and a half-bridge voltage is forced to operate at last nearly in phase with a half-bridge current; and characterized in that said oscillator input is further connected to a current supply and a capacitor, wherein said equilibrium is determined by said currently supply loading said capacitor, and said feedback circuit at least partially unloading said capacitor each half-bridge switching cycle.
 2. Ballast circuit according to claim 1 characterized in that the first end of the feedback circuit is connected to a serial connection between the two switches of the half-bridge.
 3. Ballast circuit according to claim 1, characterized in that the ballast circuit is integrated in an IC.
 4. Lamp driver for operating a gas discharge lamp using a ballast circuit, the lamp driver comprising: a half-bridge DC-AC converter having a voltage controlled oscilator for alternating switching two switches of said half-bridge, said oscillator having an input with a control voltage which determines an operating frequency of said half-bridge; a resonance circuit connected to said half-bridge for feeding the lamp; and a feedback circuit connected at a first end to said resonance circuit for adjusting the operating frequency of said half bridge, characterized in that the other end of said feedback circuit is connected to the input of said voltage controlled oscillator and designed such that during at least substantial part of a start-up period of the lamp an equilibrium exists wherein the half-bridge frequency is at least nearly equal to a resonance frequency and a half bridge voltage is forced to operate at last nearly in phase with a half-bridge current; and characterized in that said oscillator input is further connected to a current supply and a capacitor, wherein said equilibrium is determined by said currently supply loading said capacitor, and said feedback circuit at least partially unloading said capacitor each half-bridge switching cycle.
 5. The lamp driver according to claim 4 characterized in that the first end of the feedback circuit is connected to a serial connection between the two switches of the half-bridge.
 6. The lamp driver according to claim 4, characterized in that the ballast circuit is integrated in an IC. 